cmd/gc: allocate stack buffer for ORUNESTR

If result of string(i) does not escape,
allocate a [4]byte temp on stack for it.

Change-Id: If31ce9447982929d5b3b963fd0830efae4247c37
Reviewed-on: https://go-review.googlesource.com/3411Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/gc/builtin.c

@@ -33,7 +33,7 @@ char *runtimeimport =
 	"func @\"\".concatstrings (? *[32]byte, ? []string) (? string)\n"
 	"func @\"\".cmpstring (? string, ? string) (? int)\n"
 	"func @\"\".eqstring (? string, ? string) (? bool)\n"
-	"func @\"\".intstring (? int64) (? string)\n"
+	"func @\"\".intstring (? *[4]byte, ? int64) (? string)\n"
 	"func @\"\".slicebytetostring (? *[32]byte, ? []byte) (? string)\n"
 	"func @\"\".slicebytetostringtmp (? []byte) (? string)\n"
 	"func @\"\".slicerunetostring (? []rune) (? string)\n"

src/cmd/gc/esc.c

@@ -695,6 +695,7 @@ esc(EscState *e, Node *n, Node *up)
 		break;
 
 	case OARRAYBYTESTR:
+	case ORUNESTR:
 		n->escloopdepth = e->loopdepth;
 		n->esc = EscNone;  // until proven otherwise
 		e->noesc = list(e->noesc, n);
@@ -824,6 +825,7 @@ escassign(EscState *e, Node *dst, Node *src)
 	case ONEW:
 	case OCLOSURE:
 	case OCALLPART:
+	case ORUNESTR:
 		escflows(e, dst, src);
 		break;
 
@@ -1249,6 +1251,7 @@ escwalk(EscState *e, int level, Node *dst, Node *src)
 	case ONEW:
 	case OCLOSURE:
 	case OCALLPART:
+	case ORUNESTR:
 		if(leaks) {
 			src->esc = EscHeap;
 			if(debug['m'])

src/cmd/gc/runtime.go

@@ -47,7 +47,7 @@ func concatstrings(*[32]byte, []string) string
 
 func cmpstring(string, string) int
 func eqstring(string, string) bool
-func intstring(int64) string
+func intstring(*[4]byte, int64) string
 func slicebytetostring(*[32]byte, []byte) string
 func slicebytetostringtmp([]byte) string
 func slicerunetostring([]rune) string

src/cmd/gc/walk.c

@@ -1370,9 +1370,14 @@ walkexpr(Node **np, NodeList **init)
 		goto ret;
 
 	case ORUNESTR:
-		// sys_intstring(v)
-		n = mkcall("intstring", n->type, init,
-			conv(n->left, types[TINT64]));
+		a = nodnil();
+		if(n->esc == EscNone) {
+			t = aindex(nodintconst(4), types[TUINT8]);
+			var = temp(t);
+			a = nod(OADDR, var, N);
+		}
+		// intstring(*[4]byte, rune)
+		n = mkcall("intstring", n->type, init, a, conv(n->left, types[TINT64]));
 		goto ret;
 
 	case OARRAYBYTESTR:

src/runtime/string.go

@@ -185,8 +185,15 @@ type stringStruct struct {
 	len int
 }
 
-func intstring(v int64) string {
-	s, b := rawstring(4)
+func intstring(buf *[4]byte, v int64) string {
+	var s string
+	var b []byte
+	if buf != nil {
+		b = buf[:]
+		s = slicebytetostringtmp(b)
+	} else {
+		s, b = rawstring(4)
+	}
 	n := runetochar(b, rune(v))
 	return s[:n]
 }

src/runtime/string_test.go

@@ -186,3 +186,38 @@ func TestStringOnStack(t *testing.T) {
 		t.Fatalf("want: '%v', got '%v'", want, s)
 	}
 }
+
+func TestIntString(t *testing.T) {
+	// Non-escaping result of intstring.
+	s := ""
+	for i := 0; i < 4; i++ {
+		s += string(i+'0') + string(i+'0'+1)
+	}
+	if want := "01122334"; s != want {
+		t.Fatalf("want '%v', got '%v'", want, s)
+	}
+
+	// Escaping result of intstring.
+	var a [4]string
+	for i := 0; i < 4; i++ {
+		a[i] = string(i + '0')
+	}
+	s = a[0] + a[1] + a[2] + a[3]
+	if want := "0123"; s != want {
+		t.Fatalf("want '%v', got '%v'", want, s)
+	}
+}
+
+func TestIntStringAllocs(t *testing.T) {
+	unknown := '0'
+	n := testing.AllocsPerRun(1000, func() {
+		s1 := string(unknown)
+		s2 := string(unknown + 1)
+		if s1 == s2 {
+			t.Fatalf("bad")
+		}
+	})
+	if n != 0 {
+		t.Fatalf("want 0 allocs, got %v", n)
+	}
+}

test/escape2.go

@@ -1630,3 +1630,24 @@ func addstr3() {
 	s2 := s[0:1]
 	sink = s2
 }
+
+func intstring0() bool {
+	// string does not escape
+	x := '0'
+	s := string(x) // ERROR "string\(x\) does not escape"
+	return s == "0"
+}
+
+func intstring1() string {
+	// string does not escape, but the buffer does
+	x := '0'
+	s := string(x) // ERROR "string\(x\) escapes to heap"
+	return s
+}
+
+func intstring2() {
+	// string escapes to heap
+	x := '0'
+	s := string(x) // ERROR "string\(x\) escapes to heap" "moved to heap: s"
+	sink = &s      // ERROR "&s escapes to heap"
+}

test/escape2n.go

@@ -1630,3 +1630,24 @@ func addstr3() {
 	s2 := s[0:1]
 	sink = s2
 }
+
+func intstring0() bool {
+	// string does not escape
+	x := '0'
+	s := string(x) // ERROR "string\(x\) does not escape"
+	return s == "0"
+}
+
+func intstring1() string {
+	// string does not escape, but the buffer does
+	x := '0'
+	s := string(x) // ERROR "string\(x\) escapes to heap"
+	return s
+}
+
+func intstring2() {
+	// string escapes to heap
+	x := '0'
+	s := string(x) // ERROR "string\(x\) escapes to heap" "moved to heap: s"
+	sink = &s      // ERROR "&s escapes to heap"
+}